1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission, and more particularly to the control system which can ensure a control of a damper clutch and minimize a loss of drive of an oil pump.
2. Description of Related Art
Generally, an oil pump driven by a driving force generates a hydraulic pressure for engaging a friction element which changes a speed ratio by selecting a gear stage of gear stages of a multi-stage transmission gear mechanism.
A hydraulic control system pressurized by an oil pump changes ports of shift valves in accordance with an operation of a solenoid valve or proportional control valves which are controlled to be turned ON/OFF or duty by a transmission control unit.
At this point, the line pressure is supplied to the selected friction member through the changed port to engage or release the friction member.
The oil pump generates the hydraulic pressure in proportion to the number of rotation of the engine. To establish a stable speed ratio change, the hydraulic control system should receive a regular hydraulic pressure. Thus, the hydraulic pressure generated from the oil pump is always controlled in a regular line pressure by a regulator valve.
The regulator valve is designed to control the hydraulic pressure in two modes, including a driving mode and a reverse mode.
However, since the hydraulic pressure becomes a fixed hydraulic pressure at the drive and reverse modes, respectively, when the shift lever is changed from the neutral "N" range to the drive "D" range, a shift impact occurs.
Accordingly, to decrease the shift impact caused by shifting the shift lever from the neutral "N" range to the drive "D" range, an N-D control valve is employed. However, the hydraulic control system employing the N-D control valve has a problem that the drive efficiency is deteriorated since the hydraulic control system always uses the fixed hydraulic pressure.
Additionally, since a reducing valve which reduces the hydraulic pressure to be lower than the line pressure to operate a pressure control valve and a damper clutch control valve is controlled by an elastic force of an elastic member and the line pressure, the operation of the reducing valve is unreliable.
Further, a damper clutch employed for increasing a mechanical efficiency of power transmission in a torque converter is engaged or released by a damper clutch control valve. Since a position of a valve spool of the damper clutch control valve is determined in accordance with the elastic force of the elastic member and a disparity in an area of lands of the valve spool, there is a problem that the valve spool may become stuck.